Quantity   Component name
1 × 4 X 3 Phone Keypad Plenty of options, but make sure you get one with letters on it! Should be button-matrix style with 7 (sometimes 8) pins. This one has pins pre-soldered: https://chicagodist.com/products/3x4-phone-style-matrix-keypad?variant=800982981&gclid=Cj0KCQjw2NyFBhDoARIsAMtHtZ7EJEYeL_pW1BOFtWan9ynEcG3zl2Jy3wqgdJC2TMAGUySpdrGKChQaAp6UEALw_wcB
1 × Raspberry Pi Pico Pico is the cheapest option, but it does not have much flash space. Any RP2040 dev board should work!